给定一个二叉树，返回它的 前序 遍历。示例:输入: [1,null,2,3]  1\2/3 输出: [1,2,3]

代码

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/
class Solution {public List<Integer> preorderTraversal(TreeNode root) {TreeNode next=root;List<Integer> res=new ArrayList<>();Stack<TreeNode> stack=new Stack<>();while (next!=null||!stack.isEmpty()){if(next==null) next=stack.pop();//左子树已经为空，就可以遍历右子树了res.add(next.val);if(next.right!=null) stack.push(next.right);//将右子树入栈next=next.left;//先遍历到的是左子树}return res;}
}